Referring to FIG. 1, a conventional gold recovery process is depicted.
A refractory or double refractory sulfidic gold and/or silver-containing material 100 is subjected to pressure oxidation, such as in an autoclave, in step 104 to form an oxidized output slurry 108, that includes a gold and/or silver-containing residue.
The oxidized output slurry 108 is hot cured in optional step 112 to convert basic iron sulfate and free sulfuric acid to dissolved ferric sulfate and form a hot cured slurry 116.
In step 120, the hot cured slurry 116 is optionally subjected to liquid/solid separation, such as by a counter current decantation circuit, to form a washed slurry 124.
The washed slurry 124 is subjected to neutralization in step 128, typically by a weaker base such as alkali or alkaline earth metal oxides and carbonates, to neutralize most of the acid and acid equivalents in the washed slurry 124 and form neutralized slurry 132.
The neutralized slurry 132 is preconditioned in step 136 by contact with sparged air and a strong base, particularly lime, to form a preconditioned slurry 140 having a pH of about pH 8 or higher.
In step 144, the preconditioned slurry 140 is subjected to a gold and/or silver resin-in-leach process in the presence of a gold and/or silver lixiviant, such as thiosulfate, to load onto the resin the gold and/or silver in the residue. The loaded resin can be stripped and the stripped gold and/or silver recovered as a gold and/or silver product 148.
FIG. 2 depicts a conventional counter-current resin-in-leach (or resin-in-pulp) circuit 200 of the type used in step 144. The circuit 200 includes a plurality of first, second, third, . . . nth tanks 208a-n. The fresh resin 204, which is typically a strong-base anion exchange resin, is first contacted with the slurry 140 containing the lowest amount of dissolved gold, providing a driving force to promote the leaching of gold from the residue and adsorption of the dissolved gold. The gold and/or silver loaded resin 212 is removed from the first tank 208a, and barren tailings 216 are removed from the nth tank 208n. 
Although this process can be effective in recovering gold and/or silver, gold and/or silver recoveries can be problematic. Use of the resin-in-leach or resin-in-pulp method is generally limited to those gold and/or silver-bearing ores or concentrates requiring mild thiosulfate leaching conditions, since strong thiosulfate leach conditions can result in competitive adsorption on the resin by polythionate anions (e.g., tetrathionate and trithionate) produced during leaching. By way of example, tetrathionate and trithionate concentrations of 420 and 350 mg/L, respectively, have been found to reduce gold loading onto a Purolite™ A500C resin by an order of magnitude; that is, from 26 to 2 kg Au/t resin from a solution containing 0.3 mg/L Au. A typical concentration of tetrathionate and other higher polythionates in a thiosulfate leach solution ranges from about 50 to about 200 mg/L and of trithionate ranges from about 275 to about 375 mg/L.
To counter this problem, sulfite has been added to pregnant thiosulfate leach solutions in an oxygen-free atmosphere (e.g., using a nitrogen purge) to counteract the detrimental effect of polythionate concentration. Although effective, this approach can add additional expense to the process.